The present disclosure relates generally to a stator bar in a high voltage electric machine, particularly to the insulation system therein.
FIG. 1 shows cross-section of a stator bar 10 in a high voltage electric machine. It comprises a bare bar 120 and a prior-art insulation system 20 over its periphery 6. The bare bar 120 is made of two or more columns of transposed conductive strands 122, with empty spaces filled with fillers to form a rectangular section having four rounded corners. As shown in FIG. 2 and FIG. 5, the bare bar has a central straight part 123 that sits inside a slot of iron lamination core 82 and two bent-arms 6a, 6b that protrude out of the slot. All insulation systems follow this profile of the bare bar 120.
This insulation system 20 provides insulation against high voltage and simultaneously reduces partial discharges in some, but not all, air voids. Partial discharges (aka corona or void discharge) are caused by electric fields (aka electric stress) that are beyond the dielectric strength of air, so cause ionization currents, which literally burn the insulation locally and degrade its life. The prior-art of insulating a bare bar is well known, as taught for example in U.S. Pat. No. 6,420,812 or EP2333938. A representative prior-art insulation system 20 comprises four layers: (1) Internal Corona Protection (ICP) layer 22 over the periphery 6 of the bare bar 120. (2) Multiple mica layers 28 constituting a groundwall insulation 25, laid over 22. (3) External Corona Protection (ECP) layer 24, laid over 28 and engaging the iron laminations 82, with its edge 16 extending few inches out of the core (4) a Stress Grading Layer (SGL) 26 that overlaps the edge 16 of the ECP layer.
Thus, the prior-art approach is to use four layers, each made of resistive or insulative tapes with thickness ranging 0.003 to 0.015 inch, but with characteristics tailored to control discharges in specific void locations. For example, ICP layer 22 combats corona in the strand voids (crevices where two strand corners meet), transposition voids 13 (crossover areas at the top and bottom sides) and voids in the fillers. The ECP layer 24 combats corona in slot voids 14 (caused by irregular edges of iron laminations 82). The SGL layer 26 combats surface discharges from the edge 16 of the ECP layer.
At present, prior-art has no mechanism to combat the delamination voids 18 in a finished groundwall insulation 25. Such voids can occur when the mica layers 28 delaminate during operation. Further, the resistivity of the SGL layer 26 can increase 1000% during curing; it can change substantially with the field. A mismatch between ECP and SGL caused by such wide variation in resistance often results in a visible discoloration around the overlapping edge 16, a defect known as “white band” in the trade. A white band is difficult to repair and can lead to significant downtime.
Obviously, the prior-art insulation system 20 with multiple layers 22, 24, 26, 28, each tailored for specific purpose, is labor intensive, expensive and prone to defects during fabrication and operation. Its fabrication requires highly skilled labor with considerable attention to details. This necessity for tight quality control greatly increases the cost. An object of the present disclosure is therefore to provide a single layer that provides insulation against high voltages and protects itself against partial discharges in wide range of voids.
FIG. 3 shows various methods used in the prior-art (e.g., U.S. Pat. No. 6,130,496) to wind such tapes. They are known as butt-lapped, brick-lapped and half-lapped methods of winding. In the butt-lapped method 32 (aka edge-lapped), in a single row, successive tapes 42, 44 are wound side by side without gaps; a tape in the next layer (or row) piles on top of that in the previous layer, with no overlap. In the brick-lapped method 34, a row-layer comprises tapes wound side by side as in butt lap; but in the next row-layer, the tape 46 overlaps the tape 47 in previous row-layer by half-width, as in a brick-lay fashion. In the half-lapped method 36, one wraps a top tape 48 over a previous tape 49 such that an edge of the top tape 48 lies in the middle of the previous tape 49. This method results in a layer that is at least two thicknesses of the tape. Of course, a person skilled in the art could utilize various combinations of butt-lap, brick-lap and half-lap methods to achieve desired voltage resistance and flexural strength.